The exemplary embodiment relates generally to image processing systems and, more particularly, to a method of printing smooth micro-scale features.
By way of background, a printed circuit board, or PCB, is a self-contained module of interconnected electronic components found in devices ranging from common beepers, or pagers, and radios to sophisticated radar and computer systems. The circuits are generally formed by a thin layer of conducting material deposited, or “printed,” on the surface of an insulating board known as the substrate. Individual electronic components are placed on the surface of the substrate and soldered to the interconnecting circuits. Contact fingers along one or more edges of the substrate act as connectors to other PCBs or to external electrical devices such as on-off switches. A printed circuit board may have circuits that perform a single function, such as a signal amplifier, or multiple functions.
Two other types of circuit assemblies are related to the printed circuit board. An integrated circuit, sometimes called an IC or microchip, performs similar functions to a printed circuit board except the IC can contain many more circuits and components. The circuits are electrochemically “grown” in place on the surface of a very small chip of silicon. A hybrid circuit, as the name implies, looks like a printed circuit board, but contains some components that are grown onto the surface of the substrate rather than being placed on the surface and soldered.
Ink-jet printing of circuits is an emerging technology that attempts to reduce the costs associated with production by replacing expensive lithographic processes with simple printing operations. By printing a pattern directly on a substrate rather than using the delicate and time-consuming lithography processes used in conventional manufacturing, a printing system can significantly reduce production costs. The printed pattern can either comprise actual features (i.e., elements that will be incorporated into the final circuit, such as the gates and source and drain regions of thin film transistors, signal lines, opto-electronic device components, etc.) or it can be a mask for subsequent semiconductor or printed circuit board processing (e.g., etch, implant, etc.).
Several forms of printing etch masks exist. One example is that of a printed wax pattern used as a copper etch mask for creating PCBs. Another example is laser direct imaging (LDI), a maskless lithography method that is currently being used for copper etch masks on PCBs. It uses a laser to write the raster image of the pattern directly on the photoresist. In order for it to be to be cost-effective, it is necessary to have special high speed resists. Also, there is no suitable method for soldermask patterning using laser.
Typically, circuit printing involves depositing a print solution (generally an organic material) by raster bitmap along a single axis (the “print travel axis”) across a solid substrate (i.e., in the process direction). Print heads, and in particular, the arrangements of the ejectors incorporated in those print heads, are optimized for printing along this print travel axis. Printing of a pattern takes place in a raster fashion, with the print head making “printing passes” across the substrate as the ejector(s) in the print head dispense individual droplets of print material onto the substrate. At the end of each printing pass, the print head position relative to the substrate may be adjusted perpendicular to the print travel axis before beginning a new printing pass. The print head continues making printing passes across the substrate in this manner until the pattern has been fully printed.
Once dispensed from the ejector(s) of the print head, print solution droplets attach themselves to the substrate through a wetting action and proceed to solidify in place, forming printed features consisting of one spot or several connected spots. The size and profile of the deposited material is guided by competing processes of solidification and wetting. In the case of printing phase-change materials, solidification occurs when the printed drop loses its thermal energy to the substrate and reverts to a solid form. In another case, colloidal suspensions such as organic polymers and suspensions of electronic material in a solvent or carrier are printed and wet to the substrate leaving a printed feature. The thermal conditions and material properties of the print solution and substrate, along with the ambient atmospheric conditions, determine the specific rate at which the deposited print solution transforms from a liquid to a solid.
Piezo-electric ink-jet print heads can be used to obtain drops of the order of 20-60 microns. Phase change materials like wax have been used as a masking layer to pattern micro-scale features. Edge scalloping is one of the concerns in the use of jet printing for fabricating IC or other semiconductor fabrication processes. It can result in unreliable print quality and patterning defects leading to inconsistent device performance.
Edge scalloping is undesirable due to the difficulty it presents in defining the electrical performance metrics that are sensitive to edge features, such as electric resistance, electric fields, etc. It is also important to ensure the absence of short circuits that may be caused by edge scalloping in the manufacturing process since it can limit the yield.
Edge scalloping in a printed organic electronics feature may also indicate a potentially serious underlying defect. The electronic behavior of a printed organic electronics feature is affected by its molecular structure. In particular, the molecules of organic printing fluids are typically long chains that need to self assemble in a particular order. However, if a droplet of such printing solution solidifies before an adjacent droplet is deposited; those chains are not allowed to properly assemble, leading to a significant reduction in the electrical continuity or the production in electrically active defects between the two droplets. This, in turn, can severely diminish the performance of the device that incorporates the printed feature. It is therefore essential to have a method of printing adjacent drops that will promote consistent electrical properties.
The physical properties of the printed drops on the substrate govern the drop coalescence and therefore on the quality of the printed features. When a molten drop at temperature To is ejected from the print head onto the substrate, the solidification time is given by
      τ    1    =                    2        ⁢                  a          2                ⁢        k                    3        ⁢        α        ⁢                                  ⁢                  k          a                      ⁢          (                        ln          (                                                    T                o                            -                              T                a                                                                    T                f                            -                              T                a                                              )                +                              (                          1              +                                                k                  s                                                  2                  ⁢                  k                                                      )                    ⁢                      L                          c              ⁡                              (                                                      T                    f                                    -                                      T                    a                                                  )                                                        )      where Ta is the ambient temperature, Tf is the fusion temperature, α and k are the thermal diffusivity and the thermal conductivity, respectively of the molten drop and ks is the thermal conductivity of the substrate, L is the latent heat of fusion and c is the specific heat of the molten drop.
It takes additional time for the drop to cool down to the ambient temperature and the time scale for this process is given by:
      τ    2    =            2.3      ⁢              a        2            ⁢      k              3      ⁢      α      ⁢                          ⁢              k        a            The dynamics of the drop spreading on the substrate is primarily governed by the Weber number We and the Ohnesorge number Z:
            W      e        =                  ρ        ⁢                                  ⁢                  V          2                ⁢        α            σ            Z    =          μ                        ρ          ⁢                                          ⁢          σ          ⁢                                          ⁢          α                    where μ is viscosity, ρ is density, σ is surface tension, V is impact velocity and a is the radius of the drop.
The Weber number We scales the driving force for the drop spreading and the Ohnesorge number Z scales the force that resists the spreading. While impact and capillarity are the main forces for drop spreading, inertia and viscosity are the main factors that resist the drop spreading.
The time scales of the drop spreading and solidification indicate that the bulk of the drop solidifies only after the spreading is complete. However, local solidification of the drop occurs prior to the completion of the drop spreading and this determines the shape of the printed drop. The local solidification occurs at the contact line between the drop and the substrate and arrests further spreading of the drop.
When drops are ejected at a frequency f, the time between subsequent drops reaching the substrate is
  τ  =      1    f  and the distance between the centers of the subsequent drops on the substrate is
  l  =            u      f        .  Drop coalescence between adjacent drops occur when l≦2a and T≦T1.
Coalescence between adjacent drops causes lines to be formed from repetitively-deposited circular drops. Generally, substrate temperature can be increased to increase droplet spreading and promote coalescence. Typically in the case for printed wax, maintaining a substrate temperature of 30-40 degrees centigrade improves the smoothness of the printed lines. It is important that the temperature of the substrate remain lower than the freezing point of the wax so that the droplet rapidly freezes on contact with the substrate. Drop spreading on the substrate can cause widening of the printed lines. Print solutions may be engineered to have appropriately high surface tensions which can beneficially prevent the adjacent and overlapping droplet from spreading on the substrate surface, thus minimizing the lateral spreading of the droplets.
Accordingly, at least one challenge in ink-jet printing is to obtain smooth micro-scale lines in spite of the fact that the spots have circular footprints. Thus, there is a need for an improved method for printing smooth micro-scale features.